PT  - JOURNAL ARTICLE
AU  - Lim, Herman D.
AU  - Jongejan, Aldo
AU  - Bakker, Remko A.
AU  - Haaksma, Eric
AU  - de Esch, Iwan J. P.
AU  - Leurs, Rob
TI  - Phenylalanine 169 in the Second Extracellular Loop of the Human Histamine H<sub>4</sub> Receptor Is Responsible for the Difference in Agonist Binding between Human and Mouse H<sub>4</sub> Receptors
AID  - 10.1124/jpet.108.140343
DP  - 2008 Oct 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 88--96
VI  - 327
IP  - 1
4099  - http://jpet.aspetjournals.org/content/327/1/88.short
4100  - http://jpet.aspetjournals.org/content/327/1/88.full
SO  - J Pharmacol Exp Ther2008 Oct 01; 327
AB  - Using the natural variation in histamine H4 receptor protein sequence, we tried to identify amino acids involved in the binding of H4 receptor agonists. To this end, we constructed a variety of chimeric human-mouse H4 receptor proteins to localize the domain responsible for the observed pharmacological differences between human and mouse H4 receptors in the binding of H4 receptor agonists, such as histamine, clozapine, and VUF 8430 [S-(2-guanidylethyl)-isothiourea]. After identification of a domain between the top of transmembrane domain 4 and the top of transmembrane domain 5 as being responsible for the differences in agonist affinity between human and mouse H4Rs, detailed site-directed mutagenesis studies were performed. These studies identified Phe169 in the second extracellular loop as the single amino acid responsible for the differences in agonist affinity between the human and mouse H4Rs. Phe169 is part of a Phe-Phe motif, which is also present in the recently crystallized β2-adrenergic receptor. These results point to an important role of the second extracellular loop in the agonist binding to the H4 receptor and provide a molecular explanation for the species difference between human and mouse H4 receptors. The American Society for Pharmacology and Experimental Therapeutics